1. Field of the Invention
The present invention relates to an image reading method and image reading apparatus for discriminating whether or not image data obtained from an image original contains a region represented by halftone dots.
2. Description of the Related Art
When generating image data by using an image reading apparatus such as a scanner, interference fringes called moire sometimes appear in the generated image data due to the halftone dot location periodicity if an original to be read contains a photograph or figure represented by halftone dots. The occurrence of moire is suppressed by using a method by which after an original is read with a high resolution, smoothing processing is performed, and the image data is resized to the desired resolution.
Unfortunately, this method has the problem that the smoothing processing is applied to a portion such as the outline portion of a character to which it is originally undesirable to apply the processing. When successively reading a plurality of originals, therefore, the user must switch the setting for each original by determining whether or not the original contains a halftone dot region. This significantly decreases work efficiency. Accordingly, it is extremely important to establish a method of automatically discriminating whether or not an original image contains a halftone dot region, and increase the accuracy of the discrimination.
As a method of discriminating whether or not image data generated by an image reading apparatus contains halftone dots, a method that performs the discrimination by using feature pixels characteristically appearing in a halftone original has conventionally been proposed. An example of the generally used conventional techniques is a method that detects, from an image, a peak pixel having a high value as a component of interest, such as the density, with respect to adjacent pixels, and determines whether or not the target original is a halftone original based on a value calculated by using the peak pixel.
A method of discriminating a halftone original by referring to a peak pixel is disclosed in, for example, Japanese Patent No. 3,726,439. In this method, a pixel set including a peak pixel and its adjacent pixels (e.g., 3×3 pixels) is regarded as one block in target image data of a few lines, and whether the target original is a halftone original or not is determined by comparing features such as the density of the block with that of a known pattern. Another method is disclosed in Japanese Patent No. 3,183,787. This method calculates the distance between neighboring peak pixels as a peak pixel to peak pixel distance, and determines that image data is a halftone original if a specific peak pixel to peak pixel distance frequently appears in the image data.
Unfortunately, a method of calculating the distance between pixels characteristically indicating a halftone dot portion, such as the peak pixel to peak pixel distance used in Japanese Patent No. 3,183,787, and discriminating whether or not target image data is a halftone original based only on the appearance frequency has the following problem. That is, the accuracy of the discrimination is low because a feature pixel to feature pixel distance detected from a region other than a halftone dot region in an original is also taken into consideration for the determination.
In addition, even when the feature pixel to feature pixel distance of a halftone dot is constant on an original, the feature pixel to feature pixel distance is not always constant on image data depending on a combination of the number of lines of the original and the read resolution, and a high appearance frequency sometimes appears on two distances. As a consequence, a distance having a very high appearance frequency cannot be sensed, so the image data is not discriminated as a halftone original.